1. Field of the Invention:
This invention, in its preferred form, relates to apparatus for laser machining a work piece and more particularly to apparatus for focusing and directing a laser beam onto the work piece. More particularly, this invention relates to apparatus for laser machining work pieces whose focusing elements may be readily cleaned of debris thrown off during the machining of the work piece.
2. Description of the Prior Art:
The precision laser machining apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assemble 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Though not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and, therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser welding apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A to 2C. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grid strap. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grids straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22. The bent ends of the outer grid straps 22 are welded together by corner seam welds 30. Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attaches the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34 and 40 is carried out. The precision laser welding apparatus of this invention not only controls the various parameters of generating the laser in terms of the pulse width, the pulse height of each laser pulse, and the number of pulses to be applied to each weld, but also controls the sequential positioning of the fuel rod grids 16 with respect to the laser beam. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The fuel rod grid 16 is machined and in particular welded. In order to perform the intersect welds 32, the fuel rod grid 16 is incrementally moved along each of its X and Y axes, stopping at each of a plurality of positions wherein the laser beam is aligned with each of the intersections of the inner grid straps 20. Once positioned, a laser source is energized to emit a laser beam onto the aligned point of intersection to thereby effect an intersect weld 32. Thereafter, the fuel rod grid 16 is moved to the next position and another intersect weld 32 is made. The slot and tab welds 34, as well as the corner seam welds 30, are made by rotating the fuel rod grid 16 about its Y axis so that each of its outer grid straps 22 is presented to the laser beam for welding. In addition, notch seam welds 40 securing the guide sleeves 36 within the notches 38 of the inner grid straps 20 are carried out by rotating the fuel rod grid 16 to a position disposed at an angle of 45.degree. with respect to the laser beam to thereby expose the interface between the guide sleeves 36 and the slots 38 to the laser beam. It is contemplated by this invention that the laser beam is initially focused to perform the intersect welds 32 as are carried out within a single plane in which the intersect welds lie. In order to make the corner seam welds 30 and the slot and tab welds 34, it is necessary to rotate the fuel rod grid 16 out of the plane of the intersect welds 32, thus requiring the refocusing of the laser beam. In similar fashion, the fuel rod grid 16 is rotated from the plane of the intersect welds to its 45.degree. angle position with respect to the laser beam, thus also requiring a refocusing of the laser beam before precision welding may be carried out.
As described in the copending application entitled "APPARATUS AND METHOD FOR LASER MACHINING IN NON-REACTIVE ENVIRONMENT" Ser. No. 414,242 filed 9/1/82, the fuel rod grid 16 is made of a volatile material in the form of the zirconium alloy known as Zircaloy. The fuel rod grid 16 is machined and in particular welded in an environment of an inert gas to avoid contamination of the welds as would occur if the welding were to take place in the presence of a reactive medium such as oxygen or water. The danger of contaminated welds is well recognized in the art especially where the welded structure such as the fuel rod grid 16 is to be used in a hostile environment as found within a nuclear fuel reactor. Such an environment subjects the welds to relatively high temperatures in the presence of a flowing coolant such as water, whereby any weld contamination is aggravated leading to the eventual failure of the welds. In this particular example, the failure of a weld removes the structural support maintaining the spacing of the fuel rods 18, whereby the fuel rods 18 are subject to intense vibrations due to the high rate of flow of the coolant. As a result, the fuel rods 18 will rupture with the subsequent release of their uranium oxide into the coolant. Thus, it is important to carry out the machining and in particular the welding in an inert environment, while at the same time permitting the movement of the fuel rod grid 16 in three dimensions and to permit the refocusing of the laser beam onto the fuel rod grid 16 to effect different types of welds as explained above.
U.S. Pat. No. 3,422,246 of Wetzel and U.S. Pat. No. 4,190,759 of Hongo et al., disclose laser machining apparatus wherein there is relative movement between the laser source in at least X and Y dimensions with respect to a laser beam. The Hongo et al. patent particularly discloses the use of X and Y tables for receiving the work piece, i.e. a semi conductor substrate, and of a scanner for rotating the laser beam as directed onto the work piece. However, there is no disclosure within the Hongo et al. patent of machining within a non-reactive environment or of the refocusing of the laser beam because the work piece is displaced from the X and Y axes along which it is moved. The Wetzel patent discloses a platform for receiving the work piece to be machined, i.e. cut, while the laser source is moved along a Y axis transverse to the X axis. A complex beam manipulating mechanism is suggested, whereby the beam is shaped rectangularly and then rotated to maintain an edge of the rectangular beam configuration aligned with respect to the movement of the work piece.
U.S. Pat. No. 4,027,137 of Liedtke and Kelly U.S. Pat. No. 4,162,390 relate to the laser machining of a work piece in a controlled environment. The Liedtke patent suggests an assembly for supporting a spring loaded metal nozzle that engages the work piece to be welded and means for directing a jet of forced air onto the nozzle, whereby debris from the laser drilling is removed from the nozzle. To this end, the area immediately surrounding the nozzle is evacuated by a vacuum pump to further remove the drilling debris. The Kelly patent discloses a similar chamber, whereby a gas stream is directed into a chamber whose environment is controlled and is evacuated by a vacuum pump. The chamber includes a light transmitting mirror through which a laser beam is directed onto the work piece. In particular, the stream of a gas such as argon is directed onto the light transmitting mirror to remove the welding debris therefrom.
A particular problem related to this invention involves the attenuation of the laser beam by debris thrown off in the course of the laser welding of the fuel rod grid 16. As will be described below, the laser beam is focused by a lens that is disposed in close proximity to the fuel rod grid 16 to be welded and is coated during the welding by the debris thrown off from the fuel rod grid 16. Thus, the debris coating upon the lens tends to attenuate the intensity of the laser beam and thus the power imparted to a weld. It is necessary to periodically clean the laser lens quickly, while permitting the laser lens to be reinserted within the path of the laser beam at a precise point to ensure precision focusing of the laser beam.